(a) Field of the Invention
The present invention relates to an organic light emitting device (“OLED”) and a method for manufacturing the same.
(b) Description of the Related Art
Recent trends towards lightweight and thin personal computers and television sets require lightweight and thin display devices. Flat panel displays satisfying such requirements are being substituted for conventional cathode ray tubes (“CRT”).
Flat panel displays include liquid crystal displays (“LCDs”), field emission displays (“FEDs”), organic light emitting devices (“OLEDs”), plasma display panel (“PDPs”), and various other types of displays. However, among the various flat panel displays, the OLED is the most promising because of its low power consumption, fast response time, wide viewing angle, and high contrast ratio.
An OLED is a self-emissive display device which includes at least two electrodes and an organic light emitting layer interposed therebetween. One of the two electrodes injects holes into the light emitting layer and the other of the two electrodes injects electrons into the light emitting layer. The injected electrons and holes are combined to form excitons, and when the excitons de-excite they release energy in the form of visible wavelength photons.
OLEDs may be divided into passive matrix OLEDs and active matrix OLEDs depending on which method is used to drive them.
Passive matrix OLEDs include a plurality of anode lines, a plurality of cathode lines intersecting the anode lines and a plurality of pixels, each including a light emitting layer. The selection by the driving apparatus of one of the anode lines and one of the cathode lines causes light emission in the pixel located at the intersection of the selected signal lines.
Active matrix OLEDs include a plurality of pixels, each including a switching transistor, a driving transistor and a storage capacitor, as well as an anode, a cathode and a light emitting layer. The driving transistor receives a data voltage from the switching transistor and drives a current having a magnitude which is dependent on the data voltage. The current driven by the driving transistor enters the light emitting layer and causes light emission having an intensity depending on the magnitude of the current from the driving transistor.
In order to optimize the performance of the OLED, the characteristics of the switching transistor and the driving transistor are different from each other. That is, the switching transistor requires a high on/off current ratio, namely Ion/Ioff, while the driving transistor requires high charge carrier mobility and high stability for flowing sufficient and steady current to an organic light emitting diode.
When the magnitude of the off current of the switching transistor is large, a data voltage applied to the driving transistor decreases such that cross-talk between nearby pixels may occur. In addition, when the charge carrier mobility and the stability of the driving transistor are low, the current amount flowing to the organic light emitting diode reduces such that the amount of light emitted by the organic light emitting diode decreases, image sticking occurs, and the lifetime of the organic light emitting diode is reduced.
However, when the switching transistor and the driving transistor are separately manufactured through separate processes, respectively, in order to create a display which simultaneously satisfies the characteristics of the switching transistor and the driving transistor, the number of masks used in its manufacture and the costs associated therewith are increased.